This invention relates broadly to photoreceptors for electrophotography and more particularly to improved photoreceptors which are composed of amorphous silicon having a dielectric layer which increases the surface voltage in the dark while reducing residual voltage under illumination.
The photoreceptor utilizing hydrogenated amorphous silicon (a-Si:H) and a dielectric layer described above is an improvement over a photoreceptor described in my patent application Ser. No. 06/138,699, now U.S. Pat. No. 4,330,182 a continuation-in-part of U.S. Pat. No. 4,226,897 in which blocking layers were disclosed.
In the past, prior art devices described in U.S. Pat. No. 4,225,222, which issued Sept. 30, 1980, to Kempter, described p-n junctions in amorphous silicon for electrophotography. Other amorphous photoreceptors were described by Mort et al in Photographic Science and Engineering, Vol. 24, No. 5, pages 241-250. Also, UK patent application No. 2,018,466A filed Mar. 2, 1979 by Misumi et al and U.S. Pat. No. 4,265,991 which issued May 5, 1981 to Hirai, described blocking layers on imaging-forming members. Other blocking layers on amorphous silicon photoreceptors were described by Shimizu, Komatsu and Inoue in Photographic Science and Engineering, Vol. 24, No. 5, Sept. 1980, in pages 250-254. These blocking layers increased the dark surface voltage (V.sub.s) but retained substantial surface charge under illumination, thereby reducing voltage contrast.
Apparatus suitable for the present invention using asymmetrical electrical fields is described in my above-mentioned U.S. Pat. No. 4,226,897. Suitable apparatus with uniform electrical fields is described in my U.S. Pat. No. 3,069,283. Improved apparatus is described herein.
Other electro-optic devices for electrostatic printing from digital data are described by U.S. Pat. No. 3,466,657 to Rice and No. 3,471,861 to Macovski which utilize a photoconductor to switch a high voltage to an array of wires. Also, an article by Bleha entitled "Progress in Liquid Crystal Light Valves" in Laser Focus/Electro Optics, Oct. 1983, pages 111-120, describes the use of a photoconductor activated by a cathode ray tube (CRT) as a switching element for a liquid crystal light valve. In both devices the photoconductor represents the limits of performance. Space charge limited currents in photoconductors under dark and light conditions were formulated by A. Rose in Physical Review, Vol. 97, 1955, page 1531. Such electro-optic devices are improved in performance by the use of hydrogenated amorphous silicon (a-Si:H) described herein, and Rose's own analysis is helpful in understanding the underlying principles.
Finally it is relevant to note in my U.S. Pat. No. 3,068,283 mentioned above and in my U.S. Pat. No. 3,088,510 (Dec. 18, 1962) that metallic contacts were applied to films deposited by glow discharge from gases including SiF.sub.4 (i.e. fluorinated amorphous silicon a-Si:F). Chittick in the Journal of Non-Crystalline Solids, 3 (1970), pages 255-270, and Chittick, Alexander & Sterling in Journal of the Electrochemical Society Vol 116, No. 1 (1969), pages 77-81, suggested that such metallic contacts on a-Si:H form Schottky barriers, i.e. depletion of the a-Si:H under the contact and a junction current effected the polarity of the applied voltage. Metallic contacts are useful in one embodiment of my invention.